Temperature Dependence of Density of Polymer Gels 1. A Pycnometry Method Applied to Poly(N-isopropylacrylamide)−Water System

Abstract

Polymer gels that are in equilibrium with excess solvent(s) are three-component systems consisting of crosslinked polymer networks, solvent(s) in gels, and solvent(s). We have developed a method for the determination of densities of the polymer and solvent components in gels by using a specifically designed pycnometer. The method was applied to the cross-linked poly(N-isopropylacrylamide) and water system from 3.0 to 50.0 °C. From the densities of the polymer and solvent components, we have evaluated the total volume of gel per unit mass of the cross-linked polymer (solid) [V(gel)/m p ], the ratio of mass of polymer (or solvent) versus mass of gel [m p /m(gel) or m s /m(gel)], and the number of solvent molecules per polymeric residue at various temperatures. The determination revealed how contraction of the total volume of gels is related to temperature dependences of these quantities as follows: (1) The total volume at 3.0 °C [V(gel)/m p ) 32.61 cm 3 /(1.0 g of m p )] continuously decreased to a value 10.59 cm 3 at 33.5 °C. This decrease is essentially brought about by a loss in the mass of water from the gel system to liquid water; (2) The above change corresponds to the changes in m p /m(gel) and m s /m(gel) from their original values 3.055% and 96.94% at 3.0 °C to 9.417% and 90.58% at 33.5 °C, respectively; and (3) The ratio of m p /m(gel) vs m s /m(gel) asymptotically approached 4.5 vs 5.5 above 34.0 °C. The changes in both quantities caused a further shrinkage in total volume but held to less than several percent of the original value (2.017 cm 3 at 50 °C). The number of water molecules per unit polymeric residue changed from 199 (3 °C) to 60.4 (33.5 °C) and then to 7.28 (50 °C).

Transport of Ions and Electrostatic Interactions in Thermoresponsive Poly(N-Isopropylacrylamide-co-acrylic acid) Hydrogels: Electroanalytical Studies

“…His measurements verified the earlier findings that the physical state of water in gels is not identical to that of the liquid water [32]. For ionic NIPA gels modified with acrylic acid groups, he found that acrylic acid residues could retain water molecules more than 9 times that of NIPA residues.…”

“…Tokuhiro [32] has determined the number and distribution of water molecules in 15 swollen NIPA gels by measuring the densities of those gels using a specially designed pycnometer.…”

Radiolytically-induced novel materials and their application to waste processing. Final report

“…has developed a new approach for the determination of the absolute values of 1) the total volume, 2) densities of the polymer and solvent components, and 3) the stoichiometry of gels. The approach has been applied to various polymer gels (T. Tokuhiro, 1999; T. Tokuhiro, 2001). This new approach can presents results at the nanoscale and thus can compare the physical property of any gel systems composed of different kinds of polymer and/or solvent (T. Tokuhiro, 2001; T. Tokuhiro, unpublished-1).…”

“…Here, v sp (gel) and N s (gel) are the volume occupied by a single polymeric residue and associated water molecules and number of water molecules per a single polymeric residue, respectively. These studies have yielded the following results, namely that: 1) the volumetric contraction of gels with increasing temperature arise from the loss of water in gels and 2) the transition from the swollen to shrunken state is the result from the loss of water and the conformational changes in the polymer component (T. Tokuhiro, 1999; T. Tokuhiro, et al, 1991). Our DSC results for various hydrogels of poly[N-isopropylacrylamide (NIPA)] indicates that the above transition is an endothermic process with raising temperature (T. Tokuhiro, unpublished-2).…”

“…We will carry out determination as usual in water at various temperatures below and above the transition temperature. After the determination, gels must be lyophilized [by freeze drying method (Tokuhiro, 1999)] in order to determine the mass of polymer networks containing lanthanide cations. The results can be compared with the reference system, i.e., un-loaded gels.…”